A gear shift device for gear boxes for vehicles, comprising at least one pair of double-acting hydraulic cylinders, the pair consisting of a double-acting hydraulic master cylinder, whose piston can be moved by means of a gear lever, and a double-acting hydraulic slave cylinder with a piston, which via a mechanism is connected to gear selector carriers in the gear box, where the cylinder spaces in each end section of the master cylinder are connected with the respective cylinder spaces of the slave cylinder via conduits, there being provided in each end section of the master cylinder's piston a drilling, whose first end leads into the piston wall and whose second end leads into the adjacent end face of the piston, and there being provided in each end section of the master cylinder's cylinder part a drilling, whose first end leads into the cylinder wall and whose second end communicates with a fluid reservoir, in addition to which there are provided in the master cylinder non-return valves which permit fluid to flow from the reservoir to the respective master cylinder spaces.
From DK 53279 there is a known system of the above-mentioned type with return of the driven part of the system to a central or neutral position which corresponds to a neutral position of the driving part of the system, i.e. that part which is directly operated by the gear lever, where an attempt is made to find a solution in that a driving cylinder pair comprise return springs which are combined with force controlled, spring-loaded non-return valves.
The function of this system is affected by the friction which is exercised between the components, this being a disadvantage since it causes hysteresis. Furthermore, the valve device comprises many components and is complicated, which makes it more expensive.
Further, from U.S. Pat. No. 2,178,061 there is a known system in which return of the system's components to a neutral position is achieved by means of back-pressure springs on both cylinders, and from U.S. Pat. No. 2,242,542 there is a known system comprising a valve which is forcibly opened in order to achieve communication between the cylinder spaces and the tank only when the components of the device are in a neutral position.
The object of the invention is to provide a system which is not encumbered with the above-mentioned disadvantages.